Wireless communication technology, which has become a fundamental part of modern communications infrastructures, is evolving at an ever growing pace in order to meet the demanding performance characteristics of new mobile wireless communication devices. The continued increase in demand for various wireless services such as voice, data, and multimedia is also fueling the desire for higher data rates. Given that the wireless bandwidth can be expensive, technologies that improve spectrum efficiency in wireless systems are becoming desirable.
Multiple-input multiple-output (MIMO) systems can multiply data throughput, with increases also in range and reliability, without consuming extra bandwidth, thereby resulting in an improved spectral efficiency. To take full advantage of the benefits of MIMO systems, however, a design that is able to respond to the wireless channel is useful. The goal is to maximize the resources available in multiple antenna channels by using optimal schemes at all possible times.
In a typical adaptive MIMO system, some of the adjustable parameters are the modulation level, coding rate, and the transmission signaling schemes such. The performance of such an adaptive system can be superior compared to that of a non-adaptive one. However, when an inter-disciplinary analysis is performed on the interrelationships of transmission signaling schemes, antenna properties and propagation conditions, it becomes apparent that there is an additional room for further exploitation of gains of the MIMO systems. In current adaptive MIMO systems the antenna properties are fixed by the initial design and cannot be changed. It is therefore desirable to introduce an additional degree of freedom by to adaptive MIMO systems treating the antenna element properties as an additional component in the joint optimization of the adaptive system parameters. Therefore, the design of reconfigurable antennas for MIMO systems is desirable.
One use for reconfigurable antennas is in cognitive radio systems. Cognitive radio is a system in which either a network or a wireless node changes its transmission or reception parameters to communicate efficiently while avoiding interference with other users. This alteration of parameters is based on the active monitoring of several factors in the external and internal radio environment, such as radio frequency spectrum, user behavior and network state. Since the operating frequency of the a mobile wireless communications device changes frequently in cognitive radio based on detected empty spectrum, adaptive and reconfigurable antennas for these systems that are able to change their resonance frequency dynamically are desirable.